Need help on dynamic SQL

Let's say, My table structure and data sample as follow,
**************************************************
declare @tPosiHdr TABLE (
TID int not null,
BusN varchar (20) NOT NULL,
TTime varchar(6) not null,
strPosi varchar(6)not null,
DDate datetime not null
)
insert into @tPosiHdr values(1,'ADR3344','120000','00','09/01/2009')
insert into @tPosiHdr values(2,'WNR4474','130000','00','09/02/2009')
insert into @tPosiHdr values(3,'WKM1955','190000','00','09/04/2009')
insert into @tPosiHdr values(4,'WKM1955','150000','00','10/13/2009')
insert into @tPosiHdr values(5,'TXM4478','130000','00','10/16/2009')
insert into @tPosiHdr values(6,'TXM1925','170000','00','11/02/2009')
insert into @tPosiHdr values(7,'KTX1955','150000','00','11/09/2009')

declare @tPosiDtl_200909 TABLE(
TID int not null,
KT varchar (20) NOT NULL,
HPT varchar(6) not null
)
insert into @tPosiDtl_200909 values(1,'0','1')
insert into @tPosiDtl_200909 values(2,'0','1')
insert into @tPosiDtl_200909 values(3,'1','2')

declare @tPosiDtl_200910 TABLE(
TID int not null,
KT varchar (20) NOT NULL,
HPT varchar(6) not null
)
insert into @tPosiDtl_200910 values(4,'0','1')
insert into @tPosiDtl_200910 values(5,'1','2')

declare @tPosiDtl_200911 TABLE(
TID int not null,
KT varchar (20) NOT NULL,
HPT varchar(6) not null
)
insert into @tPosiDtl_200911 values(7,'1','2')
insert into @tPosiDtl_200911 values(8,'0','1')
insert into @tPosiDtl_200911 values(9,'1','2')
**************************************************

so far, i've this,
select t1.TID,t1.BusN,t1.TTime,t1.DDate,
convert(varchar(4),datepart(yyyy,t1.DDate)) + right('00'+convert(varchar(2),datepart(mm,t1.DDate)),2) as whichTable,
t2.KT,t2.HPT
from @tPosiHdr t1
left outer join
(select TID,KT,HPT
from @tPosiDtl_200909)t2
on t1.TID=t2.TID

my result will
TID| BusN |TTime | DDate | whichTable| KT | HPT
--------------------------------------------------------------------
1 ADR3344 120000 2009-09-01 00:00:00.000 200909 0 1
2 WNR4474 130000 2009-09-02 00:00:00.000 200909 0 1
3 WKM1955 190000 2009-09-04 00:00:00.000 200909 1 2
4 WKM1955 150000 2009-10-13 00:00:00.000 200910 NULL NULL
5 TXM4478 130000 2009-10-16 00:00:00.000 200910 NULL NULL
6 TXM1925 170000 2009-11-02 00:00:00.000 200911 NULL NULL
7 KTX1955 150000 2009-11-09 00:00:00.000 200911 NULL NULL

I want to make it my SQL is dynamic.

The rule is
-- If whichTable=200909, the join table will be on @tPosiDtl_200909
-- If whichTable=200910, the join table will be on @tPosiDtl_200910, and so on.

So my result will be,
TID| BusN |TTime | DDate | whichTable| KT | HPT
--------------------------------------------------------------------
1 ADR3344 120000 2009-09-01 00:00:00.000 200909 0 1
2 WNR4474 130000 2009-09-02 00:00:00.000 200909 0 1
3 WKM1955 190000 2009-09-04 00:00:00.000 200909 1 2
4 WKM1955 150000 2009-10-13 00:00:00.000 200910 0 1
5 TXM4478 130000 2009-10-16 00:00:00.000 200910 1 2
6 TXM1925 170000 2009-11-02 00:00:00.000 200911 0 1
7 KTX1955 150000 2009-11-09 00:00:00.000 200911 1 2

I think is not good to UNION ALL @tPosiDtl_200909, @tPosiDtl_200910, @tPosiDtl_200911. This way, will force the table name to be hardcoded.
It's posible my SQL statement will dynamic?
how my dynamic SQL looks like?
DECLARE @sql as nvarchar(4000)
SET @sql = ' ????'
EXEC sp_executesql @sql

Really need help

With dynamic SQL, you only need to elaborate on this bit:
SET @sql = ' ????'
T-SQL is procedural, so you can use control-of-flow stuff like IF and ELSE. You can also use variables to hold table names (but only for concatenation into a dynamic SQL statement).
In line, you can use CASE - WHEN - ELSE - END to control which bits get concatenated.

Table variables not supported in dynamic SQL, use temp. tables for more info SQL BOL.